Opal is a hydrated amorphous form of silica. Its water content may range from 3% to 21% by weight, but is usually between 6% to 10%. Because of its amorphous character it is classed as a mineraloid, unlike the other crystalline forms of silica which are classed as minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl and basalt.
Opal is the national gemstone of Australia, which produces 97% of the world’s supply. The internal structure of precious opal makes it diffract light; depending on the conditions in which it formed it can take on many colors. Precious opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare, whereas white and greens are the most common. It varies in optical density from opaque to semi-transparent. [Source]
Seen here is a stunning opal from Opal Butte, a mine in Oregon, USA (45.0438888889, -119.612222222). According to Nature of the Northwest:
“It has been more than 100 years since opal was found in Oregon. When the discovery was made public in the 1890’s miners flocked to Opal Butte… When it was discovered that the supply of material at Opal Butte was limited, commercial mining operations shut down and the site became the province of rockhounds… The land has changed hands a number of times in the past six years. The new land owners are no longer able to provide a site for digging.”
Officially known as Opal Butte opal, the beautiful gem below is owned by Inna Gem. It weighs 740 ct and measures 60 x 45 x 41 mm. When the light hits this opal it looks like an underwater scene from the ocean. For those interested, it is still available for purchase and you can contact them directly for inquiries.
[h/t malleeman on Reddit]
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